I'm afraid it is just practice, practice, practice. The guides help (I look at them every so often and want to play again :lol: ).
Also, it's worth just playing... Don't try and do anything as such, just create a huge sandbox and try out ideas.
I have one park which I just use to test out ideas and to practice with theming, scenery, ride types, etc. A few parks I made came from this, where I just went "ahhhh, that looks cool, I'd love to expand this into a slightly bigger park".
I also get inspiration from things like films. You're limited to "themes", so watch films that fit those films and take note of what things look like and atmosphere. The time travel world I did I spent a lot of time producing a kind of "lost world" ideal, very lush, lots of wood and water in with it. Most of it came from the new King Kong film though and things like "walking with dinosaurs". When you look at it and think "that's capturing the essence I want", you know you're on the right track.
Couple of major tips.
1. Challenge yourself. Start with lots of very hilly areas and then produce the park around it, modifying the land to fit what you want to do.
2. Give yourself lots of room. Don't have the ride areas touching right onto each other. You'll often swear that you can't get the coaster to turn around because it's too close to a flat ride, or whatever. You can always fill in gaps with ornamental gardens, waters features, buildings, whatever...